Time-of-flight mass spectrometers (TOF MS) are widely used in modern mass spectrometry due to their high sensitivity, mass resolving power and mass accuracy. Achieving mass resolving power in the order of 100,000 or higher at ion charge throughput >109 ions per sec and infinite mass range are typical requirements to modern TOF MS instruments. Mass resolving power of early TOF MS instruments was generally of the order of only a few hundred due to short flight times and large time spreads caused by initial spatial and velocity spreads of ions. Impressive progress in TOF mass spectrometry over the last 50+ years has at least in part been due to development of pulsed ion sources capable of generating very short ion bunches with small transverse emittances, employing elongated ion trajectories (folded between ion mirrors or multi-turn in sector fields) allowing much higher flight times and hence mass resolving power at acceptable instrument size and inventing advanced electrode geometries providing electrostatic fields with improved isochronous properties minimizing time spreads caused by optical aberrations. Simultaneously, progress in accuracy of electrode mechanical designs and particularly in development of stabilized high voltage power supplies has been useful for achieving mass accuracy of TOF MS at or under part-per-million level.
Electrostatic TOF MS instruments can in general be divided into two groups. The first group, which is the most widely used, generally employs ion mirrors to provide folded ion trajectories due to multi-reflections (MR) or a single reflection. Those are usually referred to as, respectively, MR-TOF MSs or reflectrons. The second group, which is usually noticeably smaller than the first one, generally uses electrostatic sector fields to provide single-turn or multi-turn (MT) isochronous motion of ions. In the latter case, such mass spectrometers can be referred to as MT-TOF MSs. The popularity of ion mirrors can be explained by their simpler, compared to sector fields, mechanical designs and smaller time spreads introduced by optical aberrations. Apart from purely mirror or purely sector field TOF MS's some authors have proposed hybrid instruments that include both mirrors and sector fields. Compared to purely sector field TOF MSs optical aberrations in hybrid instruments can often be minimized more efficiently.
The use of a coaxial ion mirror for compensation of energy dependency of the flight time was first proposed by Alikhanov [Alikhanov, S. G. Sov. Phys. JETP, 1956, 4, 452-453]. He also proposed to use multi-reflections to elongate the overall flight path of ions. The proposed mirror was later realized by Mamyrin in reflectron TOF MS [Mamyrin, B. A. et al. Sov. Phys. JETP, 1973, 37, 45]. Practical implementation of the idea of multi-reflections was achieved recently [Casares, A. et al. Int. J. Mass Spectrom. 206(3), 267-273] using the analyser with coaxial ion mirrors and closed reference trajectory [Wollnik, H. and Casares, A. Int. J. Mass Spectrom. 227(2), 217-222]. The idea of forming an open jig-saw trajectory folded between mirrors [Wollnik, H. UK patent GB2080021, 1981] was later applied to TOF MS systems with planar mirrors employing grids [Shing-Shen, Su. Int. J. Mass Spectrom. Ion Processes 88, 21-28, 1989] or gridless [Nazarenko, L. M. et al. USSR Patent SU1725289, 1992]. The proposed planar systems did not provide focusing in the drift direction. The drift focusing problem was solved by adding a set of focusing lenses in the drift space between mirrors [Verentchikov, A. N., et al. Patent WO2005001878] or, alternatively, providing periodic field variation in the drift direction inside planar mirrors [Verentchikov, A. N. and Yavor, M. I. Patent WO 2010/008386].
An energy isochronous TOF mass spectrometer using an electrostatic sector instead of an ion mirror was proposed by Moorman and Parmater [U.S. Pat. No. 3,576,992, 1971]. Poschenrieder considered several energy isochronous TOF MS systems using electrostatic sector fields. He also proposed to close ion trajectories into loops in a MT-TOF MS consisting of electrostatic sector fields [Poschenrieder, W. P. Int. J. Mass Spectrom. Ion Phys., 9, 357-373, 1972]. Matsuda studied TOF properties of sector fields and quadrupoles including 2nd order aberrations [Matsuda, H. et al. Int. J. Mass Spectrom. Ion Phys., 42, 157-168, 1982]. Sacurai further proposed several geometries of TOF MS systems possessing symmetry [Sacurai, T et al. Int. J. Mass Spectrom. Ion Phys., 63, 273-287, 1985] and a TOF mass spectrometer built with four cylindrical sectors [Sacurai, T. et al. Int J. Mass Spectrom. Ion Phys., 66, 283-290, 1985] Later Sakurai et al. designed and constructed a large MT-TOF MS “OVAL” consisting of six electrostatic sectors forming an elliptical closed orbit of 7.4 m [Sakurai, et al, Nucl. Instrum. & Meth. A, 427, 182-186, 1999]. Almost simultaneously, a compact MT-TOF MS “MULTUM linear plus” consisting of four cylindrical electrostatic sectors and 16 electrostatic quadrupole lenses was developed [Toyoda, M. et al, J. Mass Spectrom., 35, 163-167, 2000]. The figure-eight-shaped dosed ion orbit had a flight path length of 1.308 m per turn. A high mass resolving power of 350,000 was reported for 501.5 turns of m/z=28 ions. In the next version of the spectrometer called “MULTUM II” [Okumura, D. et al J. Mass Spectrom. Soc. Jpn., 51, 349-353, 2003] the structure was simplified by replacing cylindrical electrostatic sectors with toroidal ones having Matsuda plates [Matsuda, H. Rev. Sci. Instrum., 32, 850-852, 1961] and eliminating quadrupole lenses. The design of both the “MULTUMs” was based on the ideas of ‘perfect space and energy focusing’ [Ishihara, M. et al. Int. J. Mass Spectrom., 197, 179-189, 2000; Toyoda, M. et al. J. Mass Spectrom., 38, 1125-1142, 2003]. Several other MT-TOF MS instruments with dosed orbits were proposed by M. Ishihara [U.S. Pat. No. 6,300,625, 2001], Sh. Yamaguchi, et al [U.S. Pat. No. 7,928,372, 2011] and V. Kovtoun, et al [U.S. Pat. No. 7,932,487, 2011].
All MT-TOF mass spectrometers with closed orbits have a common drawback. After a certain number of turns ions with mass/charge ratio m1/z1 are overtaken by faster ions with m2/z2<m1/z1, which have passed more turns as compared to the ions of the first group, the effect called “overtaking”. Unambiguous identification of masses from TOF spectra in the presence of overtaking is a complicate problem. There are three main ways of solving the problem, (i) by limiting the mass range of injected ions inversely proportionally to the number of turns, (ii) by deciphering TOF spectra in the presence of overtaking and (iii) designing MT-TOF MS with an open reference trajectory (orbit). While the first approach results in very undesirable mass range limitation and the second approach has mass identification problems, the third approach of building an instrument with open trajectories does not have such problems.
The first proposal of a MT-TOF MS based on an open spiral like trajectory was put forward by Bakker in Spiratron [Bakker, J. M. B. Ph.D. Thesis, University of Warwick, 1969]. Two years earlier a simple TOF mass spectrometer with spiral trajectories was reported by Oakey and MacFarlane [Oakey, N. S., and MacFarlane, R. D. Nucl. Instr. & Meth., 49, 220-228, 1967]. In 2000 Matsuda proposed two types of TOF mass spectrometers with a corkscrew type and a mosquito-coil type open trajectories. [Matsuda, H. J. Mass Spectrom. Soc. Jpn. 2000, 48(5), 303-305, 2000]. Recently, Satoh, et al developed and built a MT-TOF MS instrument with open spiral like trajectories [Satoh, et al. J. Am. Soc. Mass Spectrom. 18, 1318-1323, 2007]. It comprises fifteen “MULTUM II” units, each having four toroidal sectors, passed by ions consecutively along a 17 m long reference orbit. Each unit is based on ion optics of “MULTUM II” with the “perfect space and energy focusing”. Mass resolving power up to 80,000 was reported. Later, an updated version of the spiral MT-TOF MS was disclosed by Satoh, et al in Patent US2011/0133073 A1. The idea of consecutive passage of ions through several isochronous units built with sector fields was also used in other proposed MT-TOF MS embodiments [Brown, J. M. Patent US 2009/0314934 and Yamaguchi, Sh. and Nishiguchi, M. Patent US 2010/0148061].
Hybrid multi-pass mass spectrometers (MP-TOF MS) including both electrostatic ion mirrors and sector fields were also proposed by some authors. Sakurai considered a MP-TOF MS with closed orbit, which additionally comprises a dipole magnet, in [Sakurai, T. and Baril, M. Nucl. Instr. and Meth. A363, 473-476, 1995]. Verentchikov and Yavor proposed a planar system with open trajectories consisting of a planar mirror and spatially isochronous sector fields [Patent WO 2006/102430]. Most recently a wider class of hybrid mass spectrometers was proposed by Verenchikov [Patent WO 2011/086430].
To provide high mass resolving power a TOF mass analyser must generally be “isochronous”, i.e. be configured to provide “isochronicity” for ions travelling along a given trajectory. The given trajectory may be open or closed.
Herein, “isochronicity” for ions travelling along a given trajectory is preferably understood as meaning that the flight time for ions travelling between two points on the trajectory is substantially independent of at least one spatial coordinate/velocity component of the ions. By substantially, it is preferably understood that mathematically the flight time is independent of said coordinates to at least the first order terms of a Taylor expansion, see below for further explanation.
Two distinct types of isochronicity are considered herein. “Spatial isochronicity” for ions travelling along a given trajectory is preferably understood as meaning that the flight time for ions travelling between two points, e.g. a start (initial) point and an end (final) point, on the trajectory is substantially independent of all the initial coordinates and velocities of the ions in a plane orthogonal to the trajectory (e.g. coordinates δy0, δz0 and velocities vy0, vz0 in FIG. 4C (Right)), unless otherwise indicated. “Energy isochronicity” for ions travelling along a given trajectory is preferably understood as meaning that the flight time for ions travelling between two points on the trajectory is substantially independent of the initial energy/velocity of the ions in the direction of the trajectory (e.g. energy=Kx0=mvx0/2 in FIG. 4C (Right)).
“Isochronicity” may exist only between two specific points on the trajectory, or may be “periodic”. “Periodic” (spatial and/or energy) isochronicity is preferably understood as meaning that the isochronicity repeats at regular (i.e. periodic) intervals on the given trajectory
Isochronicity may be achieved by adjusting (e.g. voltage settings of) electrodes based on theory, preferably calculated to at least first order terms of a Taylor expansion of the flight time with respect to initial coordinates and velocities, and possibly calculated to a second order terms of a Taylor expansion. However, once calculated theoretically, further adjustments to (e.g. voltage settings of) electrodes may be made based e.g. on empirical evidence, e.g. so as to further minimise bunch widths in the flight direction at isochronous points and/or improve the mass resolving power of the mass analyser.
FIG. 1A-FIG. 1C, FIG. 2A and FIG. 2B give examples of known mass analysers, in which ions' oscillations around the planar closed orbit are spatially and energy isochronous. Extension of the planar motion in the third direction, realized in the spiral MT-TOF MS (FIG. 2B) to retain infinite mass range, transforms the figure-of-eight dosed orbit (FIG. 1C, right) into the 3-dimensional open reference trajectory. Isochronous properties are preserved in this system.
Herein, an electrostatic sector (which can also be referred to as an “electric sector”) is preferably defined as an arrangement of at least two sheet electrodes curved in one or more directions and configured to have different potentials applied thereto so as to provide an electrostatic field therebetween for guiding ions along one or more planar or three-dimensional trajectories.
The present invention has been devised in light of the above considerations.